Stephan Klöck

Commissioning of Photon Beams of a Flattening Filter-Free Linear Accelerator and the Accuracy of Beam Modeling Using an Anisotropic Analytical Algorithm

PURPOSE To investigate dosimetric characteristics of a new linear accelerator designed to deliver flattened, as well as flattening filter-free (FFF), beams. To evaluate the accuracy of beam modeling under physical conditions using an anisotropic analytical algorithm. METHODS AND MATERIALS Dosimetric data including depth dose curves, profiles, surface dose, penumbra, out-of-field dose, output, total and scatter factors were examined for four beams (X6, X6FFF, X10, and X10FFF) of Varians TrueBeam machine. Beams modeled by anisotropic analytical algorithm were compared with measured dataset. RESULTS FFF beams have lower mean energy (tissue-phantom ratio at the depths of 20 and 10 cm (TPR 20/10): X6, 0.667; X6FFF, 0.631; X10, 0.738; X10FFF, 0.692); maximum dose is located closer to the surface; and surface dose increases by 10%. FFF profiles have sharper but faster diverging penumbra. For small fields and shallow depths, dose outside a field is lower for FFF beams; however, the advantage fades with increasing phantom scatter. Output increases 2.26 times for X6FFF and 4.03 times for X10FFF and is less variable with field size; collimator exchange effect is reduced. A good agreement between modeled and measured data is observed. Criteria of 2% depth-dose and 2-mm distance-to-agreement are always met. CONCLUSION Reference dosimetric characteristics of TrueBeam photon bundles were obtained, and successful modeling of the beams was achieved.

Clinical application of flattening filter free beams for extracranial stereotactic radiotherapy.

PURPOSE To investigate the clinical application of flattening filter free (FFF) beams at maximum dose rate for stereotactic body radiotherapy (SBRT). METHODS AND MATERIALS Patients with tumors in the lung or abdomen were subjected to SBRT using 6 MV FFF or 10 MV FFF beams. For each patient, three plans were calculated using 6 MV flattened, 6 MV FFF, and 10 MV FFF beams. Treatment times were recorded and analyzed, and tumor displacements were assessed by pre- and post-treatment cone beam computed tomography (CBCT). RESULTS Altogether, 26 patients (16 lung, 10 abdominal tumors) were treated. The average dose rate per patient ranged from 442 to 1860 MU/min. Beam-on time was on average 1.6 min (1SD=0.6 min), with the total treatment times recorded at 18.5 min (1SD=3.5 min). The time advantage of using FFF beams was dose-dependent and started at 4 Gy for 6 MV FFF and at 10 Gy for 10 MV FFF beams. The average of the tumor displacements during treatment was 2.0mm (1SD = 1.0mm). CONCLUSIONS SBRT using FFF beams is time efficient and associated with excellent patient stability. According to Van Herks formula, ITV-PTV margins of 6mm are sufficient in our patient cohort. Further studies are necessary to assess clinical outcome and toxicity.

Pretreatment quality assurance of flattening filter free beams on 224 patients for intensity modulated plans: A multicentric study

PURPOSE Pretreatment quality assurance data from four centers, members of the European TrueBeam council were analyzed with different verification devices to assess reliability of flattening filter free beam delivery for intensity modulated radiotherapy (IMRT) and RapidArc (RA) techniques. METHODS TrueBeam(®) (Varian Medical System) is a new linear accelerator designed for delivering flattened, as well as flattening filter free beams. Pretreatment dosimetric validation of plan delivery was performed with different verification devices and responses to high dose rates were tested. Treatment planning was done in Eclipse planning system (PRO 8.9, AAA 8.9). γ evaluation was performed with (dose difference) = 3% and (distance to agreement) = 3 mm scoring the gamma agreement index (GAI, % of field area passing the test). Two hundred and twenty-four patients with 1-6 lesions in various anatomical regions and dose per fraction ranging from 1.8 Gy to 25 Gy were included in the study; 88 were treated with 6 MV flattening filter free (X6FFF) beam energy and 136 with 10 MV flattening filter free (X10FFF) beam. Gafchromic films in solid water, delta(4), arccheck, and matrixx phantom were used to verify the dose distributions. Additionally, point measurements were performed using a PinPoint chamber and a Farmer chamber. RESULTS Dose calculation as well as dose delivery was equally accurate for IMRT and RA delivery (IMRT: GAI = 99.3% (±1.1); RA: GAI = 98.8% (±1.1) as well as for the two beams evaluated (X6FFF: GAI = 99.1% (±1.0); X10FFF: GAI = 98.8% (±1.2). Only small differences were found for the four verification devices. A point dose verification was performed on 52 cases, obtaining a dose deviation of 0.34%. The GAI variations with number of monitor units were statistically significant. CONCLUSIONS The TrueBeam FFF modality, analyzed with a variety of verification devices and planned with Eclipse planning system is dosimetrically accurate (within the specified limits 3 mm/3%) for both X6FFF and X10FFF beam energy.

Development and evaluation of a prototype tracking system using the treatment couch

PURPOSE Tumor motion increases safety margins around the clinical target volume and leads to an increased dose to the surrounding healthy tissue. The authors have developed and evaluated a one-dimensional treatment couch tracking system to counter steer respiratory tumor motion. Three different motion detection sensors with different lag times were evaluated. METHODS The couch tracking system consists of a motion detection sensor, which can be the topometrical system Topos (Cyber Technologies, Germany), the respiratory gating system RPM (Varian Medical Systems) or a laser triangulation system (Micro Epsilon), and the Protura treatment couch (Civco Medical Systems). The control of the treatment couch was implemented in the block diagram environment Simulink (MathWorks). To achieve real time performance, the Simulink models were executed on a real time engine, provided by Real-Time Windows Target (MathWorks). A proportional-integral control system was implemented. The lag time of the couch tracking system using the three different motion detection sensors was measured. The geometrical accuracy of the system was evaluated by measuring the mean absolute deviation from the reference (static position) during motion tracking. This deviation was compared to the mean absolute deviation without tracking and a reduction factor was defined. A hexapod system was moving according to seven respiration patterns previously acquired with the RPM system as well as according to a sin(6) function with two different frequencies (0.33 and 0.17 Hz) and the treatment table compensated the motion. RESULTS A prototype system for treatment couch tracking of respiratory motion was developed. The laser based tracking system with a small lag time of 57 ms reduced the residual motion by a factor of 11.9 ± 5.5 (mean value ± standard deviation). An increase in delay time from 57 to 130 ms (RPM based system) resulted in a reduction by a factor of 4.7 ± 2.6. The Topos based tracking system with the largest lag time of 300 ms achieved a mean reduction by a factor of 3.4 ± 2.3. The increase in the penumbra of a profile (1 × 1 cm(2)) for a motion of 6 mm was 1.4 mm. With tracking applied there was no increase in the penumbra. CONCLUSIONS Couch tracking with the Protura treatment couch is achievable. To reliably track all possible respiration patterns without prediction filters a short lag time below 100 ms is needed. More scientific work is necessary to extend our prototype to tracking of internal motion.

Ion-recombination correction for different ionization chambers in high dose rate flattening-filter-free photon beams.

Recently, there has been an increased interest in flattening-filter-free (FFF) linear accelerators. Removal of the filter results in available dose rates up to 24 Gy min(-1) (for nominal energy 10 MV in depth of maximum dose, a source-surface distance of 100 cm and a field size of 10×10 cm2). To guarantee accurate relative and reference dosimetry for the FFF beams, we investigated the charge collection efficiency of multiple air-vented and one liquid ionization chamber for dose rates up to 31.9 Gy min(-1). For flattened beams, the ion-collection efficiency of all air-vented ionization chambers (except for the PinPoint chamber) was above 0.995. By removing the flattening filter, we found a reduction in collection efficiency of approximately 0.5-0.9% for a 10 MV beam. For FFF beams, the Markus chamber showed the largest collection efficiency of 0.994. The observed collection efficiencies were dependent on dose per pulse, but independent of the pulse repetition frequency. Using the liquid ionization chamber, the ion-collection efficiency for flattened beams was above 0.990 for all dose rates. However, this chamber showed a low collection efficiency of 0.940 for the FFF 10 MV beam at a dose rate of 31.9 Gy min(-1). All investigated air-vented ionization chambers can be reliably used for relative dosimetry of FFF beams. The order of correction for reference dosimetry is given in the manuscript. Due to their increased saturation in high dose rate FFF beams, liquid ionization chambers appear to be unsuitable for dosimetry within these contexts.

Respiratory motion-management in stereotactic body radiation therapy for lung cancer – A dosimetric comparison in an anthropomorphic lung phantom (LuCa)

BACKGROUND AND PURPOSE The objective of this study was to compare the latest respiratory motion-management strategies, namely the internal-target-volume (ITV) concept, the mid-ventilation (MidV) principle, respiratory gating and dynamic couch tracking. MATERIALS AND METHODS An anthropomorphic, deformable and dynamic lung phantom was used for the dosimetric validation of these techniques. Stereotactic treatments were adapted to match the techniques and five distinct respiration patterns, and delivered to the phantom while radiographic film measurements were taken inside the tumor. To report on tumor coverage, these dose distributions were used to calculate mean doses (Dmean), changes in homogeneity indices (ΔH2-98), gamma agreement, and areas covered by the planned minimum dose (A>Dmin). RESULTS All techniques achieved good tumor coverage (A>Dmin>99.0%) and minor changes in Dmean (±3.2%). Gating and tracking strategies showed superior results in gamma agreement and ΔH2-98 compared to ITV and MidV concepts, which seem to be more influenced by the interplay and the gradient effect. For lung, heart and spinal cord, significant dose differences between the four techniques were found (p<0.05), with lowest doses for gating and tracking strategies. CONCLUSION Active motion-management techniques, such as gating or tracking, showed superior tumor dose coverage and better organ dose sparing than the passive techniques based on tumor margins.

Clinical evaluation of an anatomy-based patient specific quality assurance system

The Delta4DVH Anatomy 3D quality assurance (QA) system (ScandiDos), which converts the measured detector dose into the dose distribution in the patient geometry was evaluated. It allows a direct comparison of the calculated 3D dose with the measured back‐projected dose. In total, 16 static and 16 volumetric‐modulated arc therapy (VMAT) fields were planned using four different energies. Isocenter dose was measured with a pinpoint chamber in homogeneous phantoms to investigate the dose prediction by the Delta4DVH Anatomy algorithm for static fields. Dose distributions of VMAT fields were measured using GAFCHROMIC film. Gravitational gantry errors up to 10° were introduced into all VMAT plans to study the potential of detecting errors. Additionally, 20 clinical treatment plans were verified. For static fields, the Delta4DVH Anatomy predicted the isocenter dose accurately, with a deviation to the measured phantom dose of 1.1%±0.6%. For VMAT fields the predicted Delta4DVH Anatomy dose in the isocenter plane corresponded to the measured dose in the phantom, with an average gamma agreement index (GAI) (3 mm/3%) of 96.9±0.4%. The Delta4DVH Anatomy detected the induced systematic gantry error of 10° with a relative GAI (3 mm/3%) change of 5.8%±1.6%. The conventional Delta4PT QA system detected a GAI change of 4.2%±2.0%. The conventional Delta4PT GAI (3 mm/3%) was 99.8%±0.4% for the clinical treatment plans. The mean body and PTV‐GAI (3 mm/5%) for the Delta4DVH Anatomy were 96.4%±2.0% and 97.7%±1.8%; however, this dropped to 90.8%±3.4% and 87.1%±4.1% for passing criteria of 3 mm/3%. The anatomy‐based patient specific quality assurance system predicts the dose distribution correctly for a homogeneous case. The limiting factor for the error detection is the large variability in the error‐free plans. The dose calculation algorithm is inferior to that used in the TPS (Eclipse). PACS numbers: 87.56.Fc, 87.56.‐v

Three-dimensional versus four-dimensional dose calculation for volumetric modulated arc therapy of hypofractionated treatments.

PURPOSE Respiratory motion is a non-negligible source of uncertainty in radiotherapy. A common approach is to delineate the target volume in all respiratory phases (ITV) and to calculate a treatment plan using the average reconstruction of the four-dimensional computed tomography (4DCT) scans. In this study the extent of the interplay effect caused by interaction between dynamic dose delivery and respiratory tumor motion, as well as other motion effects were investigated. These effects are often ignored when the ITV concept is used. METHODS AND MATERIALS Nine previously treated patients with in ten abdominal or thoracic cancer lesions (3 liver, 3 adrenal glands and 4 lung lesions) were selected for this planning study. For all patients, phase-sorted respiration-correlated 4DCT scans were taken, and volumetric modulated arc therapy (VMAT) treatments were planned using the ITV concept. Margins from ITV to planning target volume (PTV) of 3-10mm were used. Plans were optimized and dose distributions were calculated on the average reconstruction of the 4DCT. 4D dose distributions were calculated to evaluate motion effects, caused by the interference of dynamic treatment delivery with respiratory tumor motion and inhomogeneously planned target dose. These calculations were performed on the phase-sorted CT series with a respiration-correlated assignment of the treatment plans monitor units (MU) to the respiration phases of the 4DCT. The 4D dose was accumulated with rigid as well as deformable registrations of the CT series and compared to the original 3D dose distribution. Maximum, minimum and mean doses to ITV and PTV, and maximum or mean doses to organs at risk (OAR), were compared after rigid accumulation. The dose variation in the gross tumor volume (GTV) was compared after deformable registration. RESULTS Using rigid registrations, variations in the investigated dose parameters between 3D and 4D dose calculations were found to be within -2.1% to 1.4% for all target volumes and within -0.8% to 1.7% in OAR. Using deformable registrations, dose differences in the GTV were below 3.8% for dose accumulation of lung and adrenal gland patients. For liver patients the used deformable registrations were not considered to be robust enough. It was also shown that a major part of the dose differences originates from the Hounsfield unit differences between 3D and 4D calculations, regardless of the interplay effect. CONCLUSION The evaluated motion effects during VMAT treatments resulted in negligible dose variability. Therefore, the approximation of calculating the dose on the average reconstruction of the 4DCT (3D dose calculation), instead of calculating on the respiration-correlated phase CTs (4D dose calculation) with assignment of the corresponding MUs, gives acceptable results.

Tumor stage, tumor site and HPV dependent correlation of perfusion CT parameters and [18F]-FDG uptake in head and neck squamous cell carcinoma

BACKGROUND AND PURPOSE This study investigated whether tumor perfusion, FDG uptake and their correlation depend on tumor stage, site and HPV in head and neck cancer. MATERIAL AND METHODS 41/55 eligible patients with integrated FDG-PET/perfusion CT from 2 prospective studies were assessed. A GTV(CT) and GTV(PET) were created. Perfusion maps were calculated using singular value decomposition method. Blood volume (BV), blood flow (BF), mean transit time (MTT) and standardized uptake value (SUV) in the tumor were compared to the surrounding tissue using Wilcoxon test and Spearman correlation of perfusion and SUVmean in the tumor was studied (p=0.05). RESULTS Perfusion parameters were significantly increased in the GTV(CT) of advanced tumors in comparison to the surrounding soft tissue (p<0.01). Oral cavity and oropharyngeal cancer showed a higher BF than laryngeal cancer (p<0.04). No correlation between perfusion and SUVmean was found, however SUVmean correlated significantly with BF for the HPV-positive tumors (r=0.86, p=0.04) and with BV for the oropharyngeal cancer (r=0.63, p=0.05). CONCLUSION Tumor stage, site and HPV are associated with different perfusion or combined perfusion/SUV signatures. Further studies are needed to investigate if these signatures co-determine clinical outcome.

TOPOS: a new topometric patient positioning and tracking system for radiation therapy based on structured white light.

PURPOSE A patient positioning system for radiation therapy based on structured white light and using off-the-shelf hardware components for flexibility and cost-effectiveness has been developed in house. Increased accuracy, patient comfort, abandonment of any skin marks, accelerated workflow, objective reading/recording, better usability and robust sensor design, compared to other positioning approaches, were the main goals of this work. Another aim was the application of a 6 degrees of freedom tracking system working without dose deposition. METHODS Two optical sensors are the main parts of the TOPOS® system (Topometrical Positioning, cyberTECHNOLOGIES, Germany). The components: cameras, projectors, and computers are commercial off-the-shelf products, allowing for low production costs. The black/white cameras of the prototype are capable of taking up to 240 frames per second (resolution: 640 × 488 pixels). The projector has a resolution of 1024 × 768 and a refresh rate of 120 Hz. The patients body surface is measured continuously and registered to a reference surface, providing a transformation to superimpose the patients surface to the reference (planning CT) surface as best as possible. The execution of the calculated transformation provides the correct patient position before the treatment starts. Due to the high-speed acquisition of the surfaces, a surveillance of the patients (respiration) motion during treatment is also accomplished. The accuracy of the system was determined using a male mannequin. Two treatment sites were evaluated: one simulating a head and neck treatment and the other simulating a thoracic wall treatment. The mannequin was moved to predefined positions, and shift vectors given by the surface registration were evaluated. Additionally manual positioning using a color-coding system was evaluated. RESULTS Two prototypes have been developed, each allowing a continuous high density scan of a 500 × 500 × 400 mm(3) (L × W × D) large volume with a refresh rate of 10 Hz (extendible to 20 Hz for a single sensor system). Surface and position correction display, as well as respiratory motion, is shown in real-time (delay < 200 ms) using present graphical hardware acceleration. For an intuitive view of the patients misalignment, a fast surface registration algorithm has been developed and tested and a real-time color-coding technique is proposed and verified that allows the user to easily verify the position of the patient. Using first the surface registration and then the color coding the best results were obtained: for the head and neck case, the mean difference between the actual zero position and the final match was 0.1 ± 0.4, -0.2 ± 0.7, and -0.1 ± 0.3 mm in vertical, longitudinal, and lateral direction. For the thoracic case, the mean differences were 0.3 ± 0.5, -0.6 ± 1.9, 0.0 ± 0.4 mm. CONCLUSIONS The presented system copes with the increasing demand for more accurate patient positioning due to more precise irradiation technologies and minimizes the preparation times for correct patient alignment, therefore optimizing the treatment workflow. Moreover, TOPOS is a versatile and cost effective image guided radiation therapy device. It allows an objective rating of the patients position before and during the irradiation and could also be used for respiratory gating or tracking.